Conventionally, semiconductor wafers (hereinafter simply referred to as a wafer) such as silicon wafers in the shape of a thin layer cut off by slicing from a silicon ingot are subjected to various kinds of processing after slicing to become final products. In the course of various kinds of processing, many or a plurality of wafers are usually laminated to obtain a wafer laminate (commonly referred to as a coin stack) and wafers are released from the wafer laminate usually one at a time and processing is applied to each wafer.
For example, an abrasive grain agent (slurry) containing oil, however, remains on and clings to the surface of the wafer as sliced from the ingot and in a similar way, a liquid such as oil, in many case, clings on the surface of the wafer subjected to various kinds of processing. In a case where many or a plurality of wafers are laminated, it is possible to move a wafer sideways but it is difficult to release a wafer upwardly from a lower wafer adjacent thereto due to the action of surface tension.
Therefore, the present inventors proposed a wafer release apparatus in which the uppermost wafer of the wafer laminate obtained by laminating many or a plurality of wafers is bended upward at the peripheral portion thereof and the uppermost wafer is raised while a fluid is blown into between the lower surface of the uppermost wafer and the upper surface of the lower wafer adjacent to the uppermost wafer to thereby release a wafer (see JP A No. 9-64152).
Description will be given of the operational principle of the conventional wafer release apparatus with reference to FIG. 11. There is pressed by a wafer pressing means 120 a center portion of the uppermost wafer W1 of the wafer laminate WS obtained by laminating many or a plurality of wafers. Then, the peripheral portion of the wafer W1 is vacuum chucked by the action of wafer vacuum chuck means 22a and 22b to thereby bend the peripheral portion of the wafer W1 upwardly. A fluid F (water and/or air) is blown into a clearance D between the lower surface of the uppermost wafer W1 and the lower wafer adjacent to uppermost wafer W, and at the same time the uppermost wafer W1 is raised to thereby release the wafer W1 from the wafer laminate WS.
Even with the conventional wafer release apparatus, wafers can be released from the wafer laminate one at a time easily and certainly, whereas a bending stress is generated in a portion indicated with a symbol S in FIG. 11, which has been led to an accident of breakage of the wafer at times.
The present inventor proposed a wafer release apparatus already in which two pairs or more (for example 4 points) of chucking positions that belong into a peripheral portion of an upper surface of a wafer and face each other with the central portion of the wafer are vacuum chucked to thereby bend the peripheral portion upwardly and thus a bending stress generated in the wafer is dispersed to thereby enable the wafer to be released safely, simply and certainly, and moreover it is devised to increase the rate for operating the release (International application number: PCT/JP02/12753). However, the apparatus still has a case of an accident of wafer breakage on which the inventor has conducted repeated serious studies in order to get more improvement, with the result that the following findings are obtained.
Wafers are obtained by growing a single crystal rod (ingot) of silicon grown by means of a CZ (Czochralski) method or an FZ (Floating Zone) method, shaping the as-grown single crystal rod into a cylindrical form with a cylinder grinding apparatus and thereafter, slicing the single crystal cylinder into thin pieces in a direction almost normal to the rod axis with wire saw or the like.
When the crystal growth of the silicon single crystal is conducted, for example, by means of the CZ method in the crystal orientation <100>, crystal habit lines L formed with the crystal planes {100} appear on the outer surface of the ingot G. Since an angle between the crystal planes {100} is 90 degrees, 4 crystal habit lines L in total are formed as ridges (linear protrusions) with a height of the order of several mm in the length direction on the outer surface of the ingot G at an angular spacing of 90 degrees when viewed in the direction of the rod axis of the ingot G.
In a wafer sliced from an ingot grown in the crystal direction <100>, for example, a disc-like wafer Wa as shown in FIG. 12(a), axes on which crystal habit lines are formed (this axis is hereinafter referred to as a crystal habit line axis), that is in the embodiment of the figure, a segmental axis indicated with a symbol A-A′ and a segmental axis indicated with a symbol B-B′, cross each other at a right angle in the central portion of the wafer, and an OF (Orientation Flat) is formed on the outer surface of the wafer so as to correspond to the crystal habit line axis. As shown in FIG. 12(b), in a wafer Wb used in fabrication of a solar cell which has been increased in production thereof in recent years, the wafer Wb itself is worked in the almost square shape and sliced such that the crystal habit line axes (in the embodiment of the figure, a segmental axis indicated with a symbol A-A′ and a segmental axis indicated with a symbol B-B′ are on diagonal lines of the wafer.
These wafers have a property that they are easy to be cleaved along a direction parallel to a crystal habit line axis. Therefore, in a case where a wafer is released from a wafer laminate, in the course of bending the peripheral portion of the wafer upwardly if a portion where a bending stress occurs and a crystal habit line axis coincide, cleavage occurs very easily; therefore, even if two or more pairs (for example, 4 points) of vacuum chuck positions facing each other on the peripheral portion of the upper surface with the central portion of the wafer are vacuum chucked and the periphery portion is bent upwardly so as to disperse a bending stress generated in a wafer as in the above-mentioned proposal, such accidents as wafer breakage are not avoidable.
When slurry remains on and clings to a surface of a wafer, which is observed just after the wafer is sliced from an ingot, water can be preferably used as a fluid blown into a clearance between the lower surface of the uppermost wafer and the upper surface of the lower wafer adjacent thereto. However, when releasing, the wafer is pulled in a direction opposite to the releasing direction due to surface tension of water. Moreover, when releasing simply upwardly, a portion where the bending stress occurs and a portion where surface tension of water works coincide each other; therefore, an accident of wafer breakage occurs easily.
If an accident of wafer breakage occurs once, as a matter of course, a yield of products decreases. Since chips from broken wafers are scattered on the wafer release apparatus, the operation is temporarily stopped and the scattered chips from the broken wafer are necessarily collected by hand, which leads to a cause of great decrease in productivity.
The present invention has been made in light of the problem and it is an object of the present invention to provide a wafer release method capable of releasing a wafer safely, simply and certainly and improving a wafer releasing rate, a wafer release apparatus and a wafer release transfer machine using the wafer release apparatus.